CHEM 1A Lecture Notes - Lecture 6: Acetic Acid, Weak Base, Strong Electrolyte

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goldclam857

14 Oct 2016

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UCSB

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Chemistry and Biochemistry

Course

CHEM 1A

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Joseph

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CHEM 1A Full Course Notes

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Solutions is a homogenous mixture of two or more substances. Solvent is the component that is in the greatest quantity: not always a liquid! Can also be a solution of metals (solid) Solute is the component that is present in lesser quantity. It is said to be dissolved in the solvent: not always a solid! Aqueous solution is a solution where water is the solvent. A compound is soluble in a particular liquid if it dissolves in that liquid: will make a homogeneous solution. A compound is insoluble if it does not dissolve in the liquid: will make a heterogeneous solution. When ionic compounds dissolve in water, they usually dissociate into their component ions: dissociate: to break apart. Water tears apart the ions in ionic compounds with its polar sides. Positive charged ion will be pulled apart by the oxygen-facing water molecules (because water is polar and oxygen is therefore the negative side)

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D. 1. Silver chloride is not soluble in water. What are the relative strengths of the intermolecular forces between Ag and Ciions and water molecules as compared to the intermolecular forces between water molecules and the forces that hold the silver chloride as a solid? a. The ionic attractions are stronger than the polar covalent bonds between the H' and O ions of the b. The ionic attractions are weaker than the intermolecular forces between the Ag' and Cr ions and water c. The ionic attractions are stronger than the intermolecular forces between the Ag' and Cr ions and water d. The ionic attractions are the same as the intermolecular forces between the Ag and CI ions and water water molecules. molecules. molecules molecules 2. The value of AHsotn for a water soluble compound is +34 kJ moÄ¾1. Which value for this compound would be numerically larger, its lattice energy or its hydration energy? Select the single best answer. step. b. The lattice energy, because it is the endothermic step c. The hydration energy, because it is the exothermic step. d. The lattice energy, because it is the exothermic step. 3, which of the following are reasons why the value of ì¨,on for a gas such as CO2, dissolving in water, is negative? a. When a gas dissolves in a liquid, there is no endothermic step as with the lattice energy of a solid. b. When a gas dissolves in a liquid, there is only an exothermic step as with the lattice energy of a solid. c. The only enthalpy change is the one associated with hydration, and this is always positive. d. The only enthalpy change is the one associated with hydration, and this is always negative. e. When a gas dissolves in a liquid the lattice energy is always higher than the hydration energy, making the net enthalpy always negative. 4. Why is ammonia so much more soluble in water than is nitrogen? a. The triple bond in nitrogen prevents it from dissolving in water. b. Nitrogen has only London forces, while both water and ammonia form hydrogen bonds. c. The molar mass of nitrogen is much larger than the molar mass of ammonia, which reduces its solubility. d. Diatomic molecules are never soluble in water s which agueous solution, if either, is likely to have the highÏr boiling point 50 m Nal or en-Na e au

Question about mixed solvents for recrystallization.

I am having a hard time conceptualizing how a mixed solvent would work in a scenario where P and O nitronitroacetanilide are recrystallized with a small amount of hot Ethanol followed by a larger volume of water.

From the structure of the molecules I assumed that the Para form of nitroacetanilide is more polar than the Ortho form. Second, I know that water>ethanol for polarity. Finally, I know that water and ethanol can hydrogen bond with one-another and with itself. Also I know that water and ethanol will hydrogen bond together to form a more densely packed liquid. I imagine that this would look somewhat like this: [ Eth-OH] --[O-H2]---[OH-Eth]

My understanding of dissolving is that the solute gets surrounded by solvent molecules and separates from other solute molecules. This happens to all the solute molecules until no more solvent molecules are left to dissolve the solute.

Having said that, does the following make sense? If not, please correct me

1) the O/P mixture of crude nitroacetanilide dissolves in hot ethanol because ethanol hydrogen bonds with the nitroacetanilide

2) when water is added, ethanol and water hydrogen bond with one another creating a denser solvent because the ethanol hydrogens are attracted to the oxygen in water and vice versa

3) because there's less hydrogens "available" to form new hydrogen bonds after the water is added, the solvent can't make new hydrogen bonds as easily

4) because the P-nitroacetanilide needs more hydrogens to dissolve due to it being more polar, it is now less soluble in the ethanol/water mixture. this causes it to precipitate out while leaving the O-nitroacetanilide that's less polar still in solution

Is this correct? if not please explain how the mixed solvent helps precipitate the p-nitroacetanilide but maintain the o-nitroacetanilide in solution.

Lab Report: Make sure your report includes the data and observations from your experiment, calculations of molar mass and a brief discussion of possible sources of error. Also include the answers to the following questions: 1. Explain the effect of each of the following errors upon your calculated molar mass. Be specific as to whether the result would be high or low and explain why. a. Some solvent is lost by evaporation. b. Some of the unknown is lost during transfer to the test tube. c. The thermometer consistently reads one degree too low. d. Part of the solute does not dissolve. 2. Is it necessary that the solvent be absolutely pure? Explain your answer clearly. 3. Many ionic compounds, e.g., NaCl, are strong electrolytes and dissociate completely upon dissolution in water. Explain clearly the effect upon the freezing point when a 1.00 m solution of NaCl in water is made (calculate the freezing point). Hint: What is the van't Hoff i-factor for this solute?

CHEM 1A Lecture Notes - Lecture 6: Acetic Acid, Weak Base, Strong Electrolyte

CHEM 1A Full Course Notes

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